Bedfordman

I have done the cable tie the heater valve shut - great in summer and cold feet in winter.

Now have the valve untied and an additional hand operated screw type tap ready on my bench.

Where is the best place to fit the hand operated tap on the S4 ? ( I would like to be able to turn it on / off without major work if possible).

merchauser

you can either remove your OEM heater valve, plug the vacuum line, and replace with your new valve, or anywhere on the firewall between your heater valve and the heater core.

I have read that it is a good idea to install an additional heater valve on the heater core return hose. I think that's a good idea.
all you will need to do is remove your air box to gain access to the valves, and open or close.

ROG100

The main reason for having to do this is the exceptionally poor quality of the generic 928 heater valve. Most are now made in China and the failure rate is horrendous. They either fall apart or the disc seal starts to leak very quickly after installation allowing hot water through to the heater matrix.
I have researched this issue fully and found that the only original style heater valve of exceptional quality is made by Mecano Bundy Gmbh in Germany. I have been selling this valve for about 3 years with ZERO issues. Works as it should every time with no leaks of hassles.
I use them on my cars here in TX and do not have to wire the valve shut in Summer and have hot water on tap when I need it.
$33.75 is a cheap price to pay for peace of mind and cold and hot WHEN you need it. As they say you pay for what you get.

hacker-pschorr

To add to what Roger said, if you still have unwanted heat the issue is elsewhere. First to check is the blend door, if that is stuck open you'll always have heat in the cabin. The other vacuum actuators add to this as well.

The vacuum solenoids that control the heather valve could have also failed.

Could also be a failing seal in the HVAC box allowing heat to escape.

So while yes, cutting off all hot water to the HVAC box is a "solution", it's better to find and fix the actual problem. I don't know about where you live, but chilly mornings and humid evenings it's still nice to have defrosters.

The HVAC system is designed to fail in "all heat" mode so at the very least you have defrosters.

Schocki

The only reason to install an additional shutoff valve is to prevent hot coolant entering the heat exchanger while the car is parked. This will make initial cooling inside the car insufficient for the first couple of minutes after engine start. This is especially true after being parked in the sun. I installed an additional valve in the return line, close to the expansion tank. I can easily open and close the valve just by sliding the air hose from the DS air filter box. No burned fingers, done in seconds.
The valve is open most of the time, only when temps rise above 30°C/90°F in the summer, I'll close it.

buccicone

I can agree with what Roger has said. Recently I got the Roger valve and it’s far superior from the generic China crap that is coming out. The China crap just does not seal. Try blowing through the valve when it’s closed you’ll be able to blow through it easily.

Bedfordman

Thanks for the comments -- agree with the quality issue: I had a Mercedes 190E in the 1990s. This used exactly the same valve but I never had this problem.

The heat soak when the 928 is parked after use is probably caused by the relative height of the heater core cp the engine cooling system. I am fitting the additional valve too.

Peter